While this would be a fantastic feature, transferring or importing third-party & external landing pages is outside the scope of Unbounce's functionality (for example, apps such as InDesign, Photoshop, Sketch, WordPress, etc.).
The page must be built in the Unbounce builder manually using our drag and drop tools.
If you're new to Unbounce, we have some great articles to get you pointed in the right direction when using our builder: Building Your Content.
Do note that you can download or upload Unbounce pages in the Unbounce app. The pages are downloaded as ".unbounce" page files, meaning they can only be shared or uploaded within Unbounce: Downloading/Uploading a Page.